Tensile test, optical and electron microscopies and EPMA were carried out on Al-Cu joints friction-welded and annealed at 350° to 500°C for 1h. An interfacial layer about 0.5μm in thickness produced between Al and Cu has a bonding strength that the joint withstands a tensile stress of 127MPa. The layer consists of CuAl
2 and grains of an unknown and probably non-equilibrium phase. Although the layer is thickened to about 1.5μm by annealing at 350°C, it still has a fine grained structure and a high bonding strength 80MPa or more which is equivalent to the ultimate tensile strength of Al annealed at 350°C. When annealed at 400°C, the layer his higher strength than Al, but cracks produced by separating two sublayers are found in the layer. When annealed. at 450° or 500°C, the layer considerably weakens, the joint breaks down at the layer under a tensile stress about 45MPa, and well-developed sublayers are observed inside of the layer. The sublayers are identified with CuAl
2, CuAl and Cu
9Al
4.
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